Optical discs, such as CD-ROM (Compact Disc-Read Only Memory) or DVD (Digital Versatile Disc or Digital Video Disc) media have data stored as a series of lower reflectance "pits" formed within a plane of higher reflectance "lands". The pits may be coated with the same material as the lands, but due to pit geometry, will diffract more light than the flatter lands, and therefore return less light to the reading sensor. The pits bottoms may be illuminated and read resembling bumps, which also diffract more light than the flatter lands. The pits are arranged in a spiral track originating at the disc center hub and ending at the disc outer rim. The data may be considered to lie in a series of tracks spaced radially from the center hub. Disc layers having no data thereon may contain a spiral groove or a series of concentric grooves, creating a diffraction pattern. Whether a spiral or a series of concentric grooves, the resulting grooves are substantially circular.
The track typically begins in a "pre-user data" area near the center hub. The pre-user area contains descriptor information of the disc itself. A "user-data" area follows the pre-user data area, and contains the data of the disc. A "lead-out" area follows the user-data area, lying closer to the outer edge of the disc. The lead-out area contains no user data, but may contain a pattern of pits which can be a repeating pattern or a random pattern having no data readout value or information content. The pre-user data area, user-data area, and lead-out area collectively comprise the "information area" of the disc.
The information content is encoded in the length of the slightly arcuate, circumferential pits. Microscopically, the track consists of a series of pits. When viewed with the naked eye, the aligned pits appear to form a continual groove, with the spiral groove forming a diffraction pattern. The diffraction pattern reflects light of varying colors, as described by Bragg's law.
In the lead-out area, the spiral track may be a continuous spiral groove, having a very long pit, formed not for information content but for appearance. Thus, while the optical disc may contain a spiral pattern to the outer rim, there may be very little user-data present, with the lead-out area comprising a majority of the disc surface in a sparsely populated disc. In even a fully populated disc, the lead-out area comprises a few millimeters near the outer rim.
A master optical disc is typically manufactured by coating a glass substrate with a positive photosensitive layer, followed by baking the coated substrate. A laser is thereafter used to expose portions of the photoresist in a spiral pattern of circumferential arcs of varying length, corresponding to the later formed pits. The master disc is thereafter developed, with the exposed areas being washed away, forming pits. The unexposed areas remain, forming a plane of higher "lands." As such, the master disc is said to be "pre-recorded" optical disc.
The master disc is used as a mold to form one or more harder, metallised, "stampers", having bumps corresponding to the pits in the master disc. The stampers are used to form the optical discs via injection molding, the final discs having pits corresponding to the bumps in the stampers corresponding to the pits in the master disc.
In CD-ROMs, the readable side includes a transparent polycarbonate bottom layer having the reflective layer thereover, which in turn has a protective "lacquer" layer thereover, which can include a label. The readable side is typically placed face down over a laser reader which penetrates the transparent layer to the single reflective layer having pits. As an example, in pre-recorded media, pits are formed in such a way to give a high contrast signal relative to the land.
DVDs are an optical disc having increased capacity relative to CD-ROMs. In one example, this is achieved in part by having double sided discs. Double sided DVDs have two reflective layers, each facing outward, requiring flipping the disc to read both sides or an additional device to read the second side. Double sided DVDs require each side to be transparent, and thus have a special labelling requirement, as covering one side with lacquer and label is no longer possible. Any label or image on the disc must be both visible, and transparent to a laser in the region containing data. Double sided discs consist of two single sided discs bonded back to back with an adhesive.
Another example of an optical disc having increased capacity relative to CD-ROMs is a dual layer disc. Dual layer discs include an innermost data layer having a reflective layer thereon, and an outermost data layer having a reflective layer thereon. The outermost reflective layer is sometimes referred to as the "semi-reflective layer" and the innermost reflective layer as the "fully reflective layer". The outermost reflective layer has optically readable differences in reflectivity between lands and pits, yet being sufficiently transparent to allow reading of the innermost reflective layer below. The desired innermost or outermost layer is optically read by focusing the optical disc reader at the proper depth for the layer of interest. For increased optical disc capacity, dual layer optical discs may also be double sided. A double-sided, dual layer optical disc includes two optically readable layers on each side of the disc.
Labelling of the previously described optical discs is important. Labelling can operate as a deterrent to piracy. The data on optical discs, being digital, can be read perfectly, and reproduced perfectly, in unlimited quantities. Standard labels on the lacquer side of CD-ROMs can also easily be reproduced, using techniques similar to those used to produce the original labels. Labelling requiring the creation of art-work on the master discs or stampers themselves would require significantly more effort and/or specialized equipment to produce copies capable of passing as the original discs.
One known method of labeling optical discs is described in U.S. Pat. No. 5,607,188, issued Mar. 4, 1997, entitled "Marking of Optical Disc for Customized Identification", the entire content of which is herein incorporated by reference. U.S. Pat. No. 5,607,188 has a common inventor and the same assignee as the present application. The above referenced application includes an optical disc which is labeled by incorporating a low contrast image, such as a "watermark", into the optical disc substrate. The image is a low contrast image which is located in the user-data area of the disc and includes optically readable data encoded therein.
It would be desirable to mark a dual sided disc in a way that would identify a disc as being authentic, that is, as having been manufactured by an authorized source. It is desirable to have a dual layer optical disc with images on one layer but not another. Further, it is desirable to have a dual layer optical disc having an image front visible on one side with the image back visible on the other side. Finally, it is desirable to create images for dual layer discs at the master disc level.